# How to identify a FRONT LAND?

I work at a town hall. I need to identify the front of the land. Here in my country we call it of "TESTADA" or "FRENTE DE LOTE". I did not find the exact term in English.

THE FRONT OF A LAND is a line near the entrance to the ground. That is, is the entry of a house. It is where the entrance gate, with the port number. To exemplify following image below.

Map before:

Map later with the front lines of terrain - As should be:

A LOT has only one FRONT OF LAND. In corners it's main street. There are old buildings that will stay as they are because it is historic. Currently we do these lines of FRONT LAND manually. We would like to automate part of it. That could be a Model Builder, Python, etc..

In short:

• In: LAYER OF LAND, LAYER STREET, BUILDING LAYER.
• Output: LAYER with FRONTS OF LAND LINES.

Another example of front lot: The term FRONT OF LAND refers to front of property.The input of the house is his front.

That would require some code, so I will point some guidelines so you can start with something and then ask more questions. The FRONT OF LAND as shown can be identified by two methods, both use spatial relationships:

Method 1) It's the only side of a polygon that does not touch any other polygon. Find the touching polygons, break the LAND polygon, check for the sides against the touching polygons for one that do no touch anything.

or

Method 2) It's the side of the polygon closest to a road. Find the closest road to the LAND polygon, break the polygon, find the side closest to the road.

This can be done with Postgis or Shapely if choose to follow the open source path. Anyway I would recommend to look at the Shapely manual to get an idea of the spatial relationships that could be tested, they are mostly the same on any software.

• what do you suggest for corner lots? – Craig Mar 19 '13 at 19:23
• To detect the corners I would suggest to use method 1, it would select the whole corner (the front and the side) with that you could then select the longest segment. (as shown in the pictures, corner lands have the front on the longest segment - edit: there is two exceptions on the top center). – Pablo Mar 19 '13 at 19:27
• judging by the photo, that is not always the case. I would suggest a third method where the front segment is along the side of the parcel where the address is derived. Of course this only is helpful if the lot has an address, and many empty lots do not. – Craig Mar 19 '13 at 19:32
• +1 That's a good idea! Maybe a mix of method 2 + address... I would suggest you to write that on an answer. – Pablo Mar 19 '13 at 19:40

Refining @Pablo's good answer, with definitions (below) and adding a check-list for each method, constrainting their use. Definitions:

Block layer is a SQL table with polygons representing the city blocks boundaries, which or without sidewalks, but preserving private (see front-1) or non-adressable alleys (see front-2) into the block.

Lot layer is a SQL table with polygons representing land lots boundaries.

Street layer is a SQL table with lines representing streets... Or, in a "river land" also rivers (and a railway can be a "front"?).

Front-1: the @Pablo's methods are about a general concept of front , where some lots have more than "one front about one street", because are at the corner (have two or more front-segments about all its surrounding streets).

Front-2: "front" is about the street indicated in the oficial address (or correspondence address) of the lot. Them, all lot (even at corner) have only one front-street-segment. PS: if your address system and block layer accept lots of an "horizontal condominuim" and its condo-addresses, the "private streets" must showed as usual streets.

The @ChristianAbreu's question illustraion show Front-2 concept (!), not the general one.

Technical note: you can plug Python with PostGIS or with any other OGC-compliant tool (for spatial SQL) to express formally the checklists.

## Check-list for method#1 - side of a polygon that does not touch any other polygon

This method have a topologic approach, is valid only for blocks where all lot's areas form their block's area. So, to use this method, you need block layer.

Expressing formallly: check if `blkarea~=sum_lotarea` for each block,

``````SELECT ST_Area(geom) INTO blkarea FROM  block WHERE gid=each1;
SELECT SUM(ST_Area(geom)) INTO sum_lotarea FROM lot WHERE gid_block=each1;
``````

Or, if the goemetries are not exact, or if the blocks are with sidewalks, use the least w for `ST_Buffer(block.geom,-w)` that ensures `ST_Within(lot.geom lot,ST_Buffer(block.geom,-w))` for all lots.

NOTE: check also

• if all lots are within a block, `ST_Within(lot.geom,block.geom)` for all lots and associated blocks.

• if, for all non-disjoint lots, there are no overlaped areas. `not(ST_Overlaps(a,b))` or this intersection area have no significance (that is `2*ST_Area(ST_Intersection(a,b))/(ST_Area(a)+ST_Area(b))<0.01`).

## Check-list for method#2 - the side of the polygon closest to a road

This method have a geometrical approach, based on minimal euclidean distance.

Check if the street layer have only "public streets", because you must to separate "public streets" (that can express the oficial address of a land lot) from "alleys" (can be internal of the lot or a "condo-block").

Then, check visually, if all "visual blocks" are surrounded by streets. If your street layer is incomplete, the method is not valid for that blocks.

# Conclusions: rules for choose methods

For Front-1 definition:

A) When you have good street data, use method#2;

B) When you have block layer and consistent lot layer, use method#1

C) When you have no good data, try split your spatial data into portions where you can use one or another method. PS: even without the block layer you can build joining neighboring lots, or by a good answer of this question.

For Front-2 definition: you need good street data, because Front-2 definition need street name (of the lot address). Use method#2 and drop front-segments that not have the same street name than the lot address.